This invention relates to circuit interrupter control systems generally and it has particular relationship to circuit interrupter timing circuits having means for avoiding circuit breaker tripping when the electrical lines to be protected have spurious or transient short overload signals or currents impressed thereon.
In the past, circuit interrupter control systems have been developed for delaying circuit breaker trip time in relationship to the amount of overload current flowing in the electrical lines being protected by the circuit breaker. The time delayed function is often known as the IT = a constant function or the I.sup.2 t = a constant function. Such protective systems are described, for example in U.S. Pat. No. 3,543,094 issued on Nov. 24, 1970, to W. H. South et al. and U.S. Pat. No. 3,590,326 issued June 29, 1971, to John D. Watson which are both assigned to the same assignee as the present application. In both cases, relatively complicated variable long time delayed tripping circuits are provided which are specifically aimed at providing a tripping characteristic which is accurately related to a current-time tripping function, such as previously described.
Means have been disclosed in the prior art for controlling the tripping characteristic of a static time overcurrent relay or similar means by the use of resistive-Zener diode networks, such networks are described in U.S. Pat. Nos. 3,434,011 issued Mar. 18, 1969 and 3,444,434 issued May 13, 1969 which were issued to S. E. Zocholl. In those patents, the use of Zener diodes and resistive networks for tripping purposes was aimed at shaping a time versus current curve. The shaping is meant to deviate or change the tripping characteristic of the circuit breaker from a relatively precisely shaped curve to a different, but nevertheless, relatively precisely shaped curve. The teachings of South and Watson in the previously mentioned U.S. patents are aimed at a relatively precise control of the time that a circuit breaker trips in relationship to the amount of overcurrent flowing in the line to be protected. It would be advantageous, however, to slightly delay the tripping or current interruption of a circuit breaker in relationship to the current flowing in lines to be protected in order to avoid spurious or transient or line surge tripping of the circuit breaker which might be caused by high level short duration currents in the line to be protected or the inrush of current associated with connecting incandescent lamp loads into a circuit. It should be realized that, in some instances, relatively low levels of overcurrent, that is overcurrents between 500 and 1000% of rated current which last for a very short period of time should not cause a circuit breaker to trip, because the line to be protected in most instances can withstand these energy levels without damage to components. It would be very advantageous to be able to do this inexpensively and simply without relying on mathematical function generators or the like to determine the exact tripping characteristic desirable or necessary. In the teachings of Zocholl, relatively precise wave shaping is accomplished by the use of Zener diodes and resistors used in a common network. However, in those cases, the wave shaping is provided to change the characteristic current versus time curve. It would be advantageous to use a resistive network in conjunction with a Zener diode to provide nearly instantaneous tripping of a circuit breaker if electrical current exceeds a given predetermined amount which, for example, could be 1000 percent of rated current.